A catalytic converter exhaust-gas after treatment is required in order that the toxic substance emissions in the form of hydrocarbons, carbon monoxide and nitrogen oxide remain below the statutory limit values therefor during operation of motor vehicles. The catalytic converter reduces the active energy for the chemical reaction which converts the toxic substances into harmless end products such as water, carbon dioxide and nitrogen. An effective exhaust-gas after treatment still requires catalytic converter temperatures of at least a so-called light-off temperature of 250° to 300° C. The objective of the exhaust-gas optimization is therefore to bring the catalytic converter as rapidly as possible to the light-off temperature and to emit as little toxic substances as possible because here there is still no after treatment possible. The exhaust gas of the motoric combustion functions as a heat source for the catalytic converter and the catalytic converter is additionally heated up for this purpose via a targeted deterioration of the degree of efficiency of the internal combustion engine. If this is not sufficient for reaching the desired exhaust-gas standard, then additional heating measures are required utilizing external components.
A proven method provides for blowing in fresh air into the exhaust-gas system which is characterized hereinafter as secondary air. The secondary air reacts exothermally with the excessive fuel from the motoric combustion starting from an ignition temperature of approximately 600° C. The advantages of this after combustion are, on the one hand, a clear increase of the exhaust-gas temperature which is needed for the warming of the catalytic converter to operating temperature and, on the other hand, for a reduction of the hydrocarbon emissions because of the combustion process.
The secondary air blow-in is only active until the catalytic converter has reached its operating temperature. For diagnostic purposes, the secondary air blow-in is then activated once more for a short time during the driving cycle.
For the in-blow of secondary air, an electric secondary air pump was used up to now. In this way, the further influence of the secondary air blow-in on the engine control is essentially limited to a correction of the control of the air/fuel mixture ratio and an increase of the torque reserve because of the additional generator load required by activating the electric pump. This method of rapid catalytic converter heating is achieved with a current consumption by the secondary air pump of up to 80 A during run-up and approximately 32 A in continuous operation as well as a weight of approximately 2.5 kg and a volume of approximately 3 liters.
A so-called secondary air charger is known from the publication entitled “Secondary Air Charger-high integrated secondary air system for intake manifolds” by K. Hummel and S. Wild, SAE paper 2001-01-0665. This secondary air charger utilizes the pressure drop across the throttle flap in the air supply to the internal combustion engine in order to drive a turbine. The turbine is coupled by a shaft to a compressor for compressing the secondary air which is blown into the exhaust-gas system in a conventional manner. The advantages of such a secondary air charger are the following: the absence of a need for electric drive energy; a relatively low weight of approximately 0.5 kg at the present time; and, a comparatively low volume of approximately 1 liter at the present time. The secondary air charger is controlled via a valve which releases the air mass flow for the drive of the secondary air charger.